FR2709854A1 - Visualization device with optimized colors. - Google Patents

Abstract

The device of the invention comprises a source of white images (2), a liquid crystal matrix (5) for controlling the hue, and a liquid crystal matrix (6) for controlling the saturation of the displayed images. .

Description

OPTIMIZED COLOR DISPLAY DEVICE

  The present invention relates to a display device

with optimized colors.

  The visual aspect of any point of a color image is entirely defined by three independent parameters: luminance, hue and saturation, these last two parameters representing the color of the point considered. The performances of human vision relating to variations in luminance are very different from those relating to variations

  spatial and temporal of color.

  Conventional display systems do not allow luminance and color to be controlled separately, and therefore

  misuse the difference in perception between luminance and color.

  To synthesize a color image, there are two methods

  the so-called additive method and the so-called subtractive method.

  The systems implementing the additive method modulate the respective levels of light power of three distinct radiations of fixed color, and mix them. These three distinct colors are called primary colors, and are for example red, green and blue. Several techniques for taking into account the colors of neighboring elements in images are known for carrying out the additive mixing: spatial average of juxtaposed elements, time average,

spatial overlay.

  The visual appearance of each picture element is controlled by modifying the light levels of each of the three primary light radiations. The relative values of these three primary light levels give the color of each picture element. The sum of these three levels gives the luminance of each picture element. To modify only the luminance of an image element without modifying its color, it is necessary to modify the light level of the three primary radiations which compose it. Likewise, to modify only the color, without modifying the luminance, it is necessary to modify the light level of these three primary radiations. In systems implementing subtractive synthesis, the three modulated parameters are the respective transmission levels of three bandpass filters of fixed color successively crossed by an originally white radiation. These three distinct colors are, for example, the complementary colors of the three primary colors

  mentioned above, that is to say cyan, magenta and yellow.

  The visual appearance of each picture element is controlled by modifying the transmission level of each of the three filters. To modify only the luminance of an image element without modifying its color, the transmission level of the three color filters must be modified. Likewise, to modify only the color, without modifying the luminance, it is necessary to modify the transmission level of the three colored filters. Another disadvantage of subtractive synthesis systems is that the maximum value of the transmission is limited by the fact that each filter absorbs

  always at least one spectral band.

  The control of color image synthesis systems, whether additive or subtractive, therefore consists in using several spectral bands of determined positions on the wavelength scale, in dosing them in variable proportions and in add or subtract them from the incident radiation from the light source. These systems do not allow direct control of color without changing the luminance, or direct control of the luminance without changing the color. One consequence is that the definition of luminance images is equal to their definition in color, that is to say that for a given image, the maximum number of image elements of any luminance is equal to the number d elements of any color, or even that the maximum spatial frequency of modulation of luminance, with constant color, which the system can reproduce correctly, is equal to the maximum spatial frequency of modulation of color, with luminance

  constant, which the system can reproduce correctly.

  However, the performance of human vision, in particular in terms of spatial frequencies, is clearly different from that of known image synthesis systems: visual acuity is much better in luminance than in color. Therefore, known systems are poorly

  adapted to the performance of human vision.

  The present invention relates to a device for viewing color images which can be optimally adapted to the performance of human vision, and which can be adjusted to the vision of

each user.

  The display device according to the invention comprises, in cascade, a device for controlling the luminance of the displayed images, a device for controlling the tint of these images, and a

  device for controlling the saturation of these images.

  The present invention will be better understood on reading the

  detailed description of several exemplary embodiments taken as

  of nonlimiting examples and illustrated by the appended drawing, in which: FIG. 1 is an exploded and simplified view of a first embodiment of the invention, with cathode ray tube, polarizer and two control devices; - Figure 2 is an exploded and simplified view of a second embodiment of the invention, similar to the first embodiment, but with a polarizer replaced by a control device; - Figure 3 is an exploded and simplified view of a third embodiment of the invention, similar to the first, but the cathode ray tube of which has been replaced by a matrix optical valve, backlit by a light source, and - Figure 4 is an exploded and simplified view of a fourth embodiment of the invention, similar to the first, but with devices

conjugation optics.

  There is shown in Figure 1, in a very simplified manner, the display device 1 according to the invention. This device comprises, from the image source 2 to the eye 3 of the observer, a polarizing device 4, a device 5 for controlling hue and a device 6 for controlling saturation. The polarizer 4 is for example a spectrally neutral linear polarizer, but it could be a spectrally neutral circular polarizer. It should however be noted that the order of the devices 4 to 6 is not necessarily that indicated above, and that they can be

  swapped with each other in most embodiments.

  The source 2 provides, in a manner known per se, a white image (with gray levels) polarized by the polarizer 4. The device 5 is an optical liquid crystal valve which distributes the radiation of the source 2 according to two axes of polarization perpendicular between them and according to two complementary colors whose hue (dominant wavelength) depends on the

  control voltage applied to the light valve.

  The device 6 is a liquid crystal optical valve which removes a variable fraction of the radiation it receives, for fear of one of the two aforementioned polarization axes (or for a polarization direction if the

  polarizer 4 is circularly polarized).

  Thus, the device 2 acts only on the luminance, while the devices 5 and 6 act essentially on the color and partially on the luminance: the device 5 acts on the hue (or wavelength

  dominant) and device 6 acts on saturation.

  Thus, the elements 2 + 4 provide a polarized white image by acting only on its luminance. Element 5 distributes this radiation on two perpendicular polarization axes (x and y) and according to two complementary colors whose hue (or dominant wavelength) depends on the control voltage of this element. Element 6 removes a variable fraction of the radiation, which it receives from 2 + 4 through 5, along one of the two aforementioned polarization axes. Elements 5 and 6 act essentially on the color of the displayed image, and partially on its luminance. In the example described, element 5 acts on the hue (or dominant wavelength), while element 6 acts on the saturation of the images displayed. Consequently, the device of the invention makes it possible to separately adapt the characteristics of each of the electro-optical devices 2, 5 and 6 so as to optimize the image formed by the display device 1 with respect to the characteristics of human vision. This optimization consists in adjusting separately for the luminance, for the hue and for the saturation: the contrast, the definition, the number and the spacing of the quantization levels of the different points of the displayed image, the refresh rate and the time. rise of control signals

image settings.

  The device of the invention makes it possible, in particular, to avoid the "over-definition" of the color parameter. Thus, for example for an image defined in a conventional manner, on N lines of N pixels per line, it is necessary to modulate 3N2 pixels. We can very well, without altering in a perceptible way to the eye the quality of the images viewed, divide by two, on each axis (lines and columns), the definition of the hue and the definition of the saturation, which

  allows to have to modulate only 1.5N2 pixels.

  In addition, this reduction in the definition of the optical valves modulating the hue and the saturation significantly increases their optical transmission and therefore improves the energy efficiency of the display device. The cathode ray tube 2 is of the white phosphor type, and the image to be displayed is formed on its screen. This cathode ray tube emits white light spatially modulated in intensity. This light contains the three primary colors red, green and blue. Let L (X) be the luminance

  spectral of one of the light rays emitted by this tube 2.

  The polarizer 4 is for example a neutral linear polarizer, acting along the Y axis (vertical). This polarizer absorbs half the light energy of tube 2. The spectral luminance of the radiation transmitted by

polarizer 4 is 0.5 L (k).

  The effect of the matrix 5 is to modify the spectral distribution of light energy at each of the crossing points of its network along the two perpendicular polarization axes x and y of the matrix, as a function of the potential applied to each of these points. The matrix 5 comprises molecules of liquid crystals whose orientation is individually controlled electrically, by controlling the electric potential applied to the respective electrodes relating to the crossing point considered. The axis of these molecules represents the axis

  "extraordinary" of the liquid crystal layer.

  For an incident light energy polarized (by the polarizer 4) along the y axis, and having a luminance of 0.5 L (X), the light energy at the output of the matrix 5, along each axis of polarization, is given by: Lx (A) =: 0.5 L (A) sin2 (k / A) Lv (A) = 0.5L (A) cos2 (k /) with: L (;) = spectral luminance emitted by the cathode ray tube, X = wavelength, Lx (X): spectral luminance along the x axis, Ly (x) = spectral luminance along the y axis, and k = coefficient which is a function

  of the potential applied to the pixel considered in the matrix 5.

  This phenomenon of the modification of the spectral distribution of the incident light energy is due to a nematic liquid crystal, in which the axis of the first molecules encountered by the light rays entering the matrix 5 is located at 45 from the y axis. (the axis of molecules

  following, in the thickness direction, being at an angle other than 45).

  When no potential is applied, the coefficient k is a function of the product of the thickness of the liquid crystal cell and the value of its birefringence. When a high potential is applied to the cell, the axis of the liquid crystal molecules aligns with the direction of light propagation (z-axis). The plane of polarization of the light is unchanged and

  corresponds to a zero value of k.

  For intermediate values of potentials applied to the cell, k takes values between zero and the aforementioned value

obtained without potential.

  Because the light emitted by the cathode ray tube 2 is white, the colors of the light energies respectively associated with the axis of

  polarization x and the axis of polarization y, are complementary.

  A matrix such as matrix 5 works on the principle

  commonly known as electrically controlled birefringence.

  The liquid crystal matrix 6 has the effect of analyzing light (like a so-called "analyzer" filter) with an electrically controllable polarization rate. When no potential is applied to the matrix 6, the light energy transmitted by this matrix is that of the axis of polarization y. When a high potential is applied to the matrix 6, it fully transmits the light energy, and the tint of the image formed

  by tube 2 and observed downstream of matrix 6 is that of tube 2, that is to say

  say white. For potentials of intermediate values applied to the matrix 6, the light energy is entirely transmitted on the y axis and partially on the x axis. For this x-axis, the transmitted light energy is given by the relation: Li (A) [a (À) Lx (,)] + Ly () relation in which a (V) is a transmission coefficient between 0 and 1, as a function of the potential V applied to the matrix 6, with a (0) = 0 and

a (x) = 1.

  Thus, when the potential V is varied, the set of colors between white and the color associated with the axis of polarization y is scanned. This phenomenon can be explained as follows. The liquid crystal contains dye molecules, generally spectrally neutral, aligned along the x axis and absorbing light energy along this axis. The transmission coefficient of the molecules is modified by tilting their axis in the direction of the propagation of light using the electric potential applied to the matrix. Such a

  matrix is of the type known as "nematic with dyes" ("guest host" in English).

  According to a second embodiment of the invention, shown diagrammatically in FIG. 2, a device is used which, like that of FIG. 1, a cathode ray tube 2 with white phosphors, a first matrix 5 and a second matrix 6, but instead of the polarizer 4 a third matrix 7 is used. This third matrix 7 is identical to the matrix 6, that is to say it is a variable polarization rate matrix. The transmission of the matrix 5 in depolarized light is spectrally neutral. If we removed the matrix 7 and if we ordered the matrix 6 in non-polarizing mode (high potential applied to the matrix 6), all of the matrices 5 and 6 would have an overall transmission of 100% and a spectral behavior neutral Therefore, with the matrix 7, controlled at variable polarization rate, the transmission of the light energy emitted by the tube 1 can be improved compared to the case of the device of FIG. 1, for a neutral spectral behavior. In the case of two identical matrices 7 and 6 modulated by the same voltage V, the light energy Lt transmitted by all of the matrices 7, 5 and 6, as a function of the variable transmission coefficient of these matrices, is then given by the following relation: Lt () O =, 5L (À) [(lpa 2 (i)) cos2 (k / A) + 2a12 (V) sin2 (k / A)] In this relation, the coefficient a (V ) is a function of the state of the two matrices 7 and 6 with variable transmission coefficient (the transmission coefficient is variable since the polarization rate is varied). This

  coefficient a (V) varies between 0 and 1.

  With this device of FIG. 2, the transmission of light energy is doubled compared to the case of FIG. 1, when the color of the images downstream of the matrix 6 is identical to that of the images produced by the cathode ray tube 2 (color white in general), and we then have a (V) = 1. For these slightly saturated colors, we clearly increase, the overall transmission of the display device due to the fact that the energy from the source is recovered almost completely on one of the polarization axes, and partially on the other, depending on the degree of color saturation. For saturated colors, ie those for. which has (V) = 0, that is to say those which are the furthest from white with regard to saturation, the transmission of light energy is not

  decreased compared to the first example.

  For the intermediate colors (between white and full saturation), the transmission rate of all the matrices is, compared to the first example, multiplied by a factor between 1 and 2, in

  depending on the saturation of the displayed color.

  According to the exemplary embodiment of FIG. 3, which may include the set of matrices 7, 5, 6, as shown in the drawing, the cathode ray tube is replaced by a matrix optical valve 8 with spectrally neutral liquid crystals. This valve can for example be a helical nematic liquid crystal matrix, associated with a neutral linear polarizer 9 which can precede it. Preferably, this matrix 8 is backlit by a light source 10, preferably white,

  advantageously through a collimator 11.

  Since in the embodiment of FIG. 3 the image source 8 is a spectrally neutral optical valve, it is possible to completely reverse the order of the elements, and instead of having, in the order described above above the elements 10, 11, 9, 8, 7, 5, 6 we can have: 10, 11, 6, 5,

  7, 8, 9 (element 9 then being closest to the observer).

  In the embodiments described above, the essential elements of the display device (image source, matrices and polarizer, if any) are very close to each other, and can

  even be glued to each other.

  It is however possible, as shown diagrammatically in FIG. 4, to separate them notably from one another and to couple them

  optically using conjugation optics.

  In the example of FIG. 4, the same elements have been used as in FIG. 1, namely elements 2, 4, 5 and 6, the eye of the observer being referenced 3. It is however understood that one could also use the elements of the other embodiments described above, and reverse

  the order of these elements, as specified above.

  Only the polarizer 4 and the tube 2 are close to each other or joined to one another. The elements 5 and 6 are spaced apart and relative to the polarizer 4. A first conjugation optic 12 is arranged between the elements 4 and 5, and a second conjugation optic 13 is disposed between the elements 5 and 6. The optic 12 conjugates the image source 2 with the matrix 5, while optics 13 conjugates element 5 and the image of source 2 with element 6. The conjugation optics can be conventional conjugation optics with magnification of 1 or different from 1 (for example conventional lenses of cameras) or any suitable device for transporting images, for example with optical fibers, which is advantageous for example when the different matrices have to be not far apart each other. Of course, one of the two abovementioned conjugation optics (12 or 13) can be omitted if the elements between which it could have been

  arranged are very close to each other.

  If the position of the observer relative to the display device described above varies little in all cases of use,

  The observer can directly observe the image produced by this device.

  On the other hand, if the position of the observer can vary significantly, there is a translucent screen 14 in front of the device described above, and a conjugation lens 15 between the element 6 and the screen 14 (the elements 14 and 15 are shown in broken lines in Figure 4). The display device comprising the additional elements 14 and 15 also has the following advantage. The incidence of light rays (relative to the screen 14) which participate in the formation of the image observed by the observer 3 does not vary, on the path downstream of the screen 14, from the

  source 2, when the position of the observer relative to the screen 14 varies.

  This thus removes any misalignment (parallax error) between the corresponding pixels of each of the elements 2, 5 and 6 when the position of the observer varies (it being understood that the elements 2, 5 and 6 are well aligned and that the optics 12, 13 and 15 are correctly placed), and we also remove any color variation on the pixels of each of the

  elements 5 and 6 when the position of the observer varies.

  Thus, the device of the invention can be compact (embodiments of Figures 1 to 3), while having a high optical transmission rate.

Claims (15)

  1. A color display device for optimizing the parameters of the images, in particular with respect to human vision, characterized in that it comprises, in cascade, a device for controlling the luminance of the images displayed (2, 10- 11), a device for controlling the hue of these images (5), and a device for controlling the the saturation of these images (6).   2. Device according to claim 1, characterized in that the device for controlling the luminance of the images displayed is a tube cathodic (2).   3. Device according to claim 2, characterized in that the   cathode ray tube is of the white phosphor type.   4. Device according to claim 1, characterized in that the luminance control device comprises an optical valve matrix (8).   5. Device according to claim 4, characterized in that the   optical valve is spectrally neutral.   6. Device according to claim 4 or 5, characterized in that   that the optical valve is of the helical nematic liquid crystal type.   7. Device according to claim 6, characterized in that the   optical valve is associated with a neutral polarizer (9).   8. Device according to one of claims 4 to 7, characterized by   the fact that the valve is backlit by a collimated source (10) (11).   9. Device according to claim 8, characterized in that the   source is a white light source.   10. Device according to one of the preceding claims,   characterized in that the devices for controlling the hue and the saturation of the images each comprise a liquid crystal matrix (5,   6) electrically operated with variable voltage.   11. Device according to claim 10, characterized in that it   further includes a spectrally neutral polarizer (4, 9).   12. Device according to claim 10, characterized in that it further comprises a third liquid crystal matrix (7) at a rate of variable polarization.   13. Device according to one of the preceding claims,   characterized by the fact that it includes at least one conjugation optic   (12, 13) associated with the hue and saturation control devices.   14. Device according to one of the preceding claims,   characterized by the fact that it comprises a translucent screen (14).   15. Device according to claim 14, characterized in that   the screen is associated with a conjugation optic (15).